The artist has skilfully realised the oppressive and enervating atmosphere; and it was till lately quite usual to see in the side streets of Paris in the early morning the boulanger at work precisely in the same informal costume.

Not being a writer of novels, I can't answer your question. I, for one, don't know what the French word is for baker, but I could guess it if I'd read the entire paragraph. The rest of the paragraph might illuminate the writer's choice:

"He could see her heart in her throat the first time she spoke French to a real Frenchman; her pulse fluttered in the hollow of her neck like a trapped hummingbird. But the boulanger (etc., below)... at the same time.

'He understood me!' she said, clutching him by the arm as they left... 'I spoke French to him, and he kent what I said, clear as day!'"

I'd argue that the word is given here in French to give some flavor of the language, just as the dialogue of the characters (who are Scottish) also gives a flavor of their accent.

In the citation below, why not just say baker? Or why not use the French words for foreigner and accent as well? Why do writers use foreign nouns like this, indiscriminately, when there is a perfectly serviceable English translation?

"But the boulanger understood her—Brest was full of foreigners, and her peculiar accent roused no particular interest—and the sheer delight on her face when the man took her penny and handed her a baguette filled with cheese and olives made Jamie want to laugh and cry at the same time."